The present invention relates generally to reaction bonded silicon carbide articles and to methods of preparation thereof and more particularly to silicon carbide-graphite articles of manufacture and to methods of producing such articles. The invention is a result of a contract with the Department of Energy (W-7405-ENG-36).
In ceramic matrix technologies, there has been a need for high performance structural ceramic composites made of silicon carbide (and optionally including some graphite). These have been needed, for example, for liners of fusion reactors and for turbine blades and stators. Other applications have been for tubular heat exchangers, recuperators, and regenerators. However, because silicon carbide is a refractory material, it is made into complex shapes with great difficulty.
Whiskers (i.e., fibers which have been grown under controlled conditions that lead to the formation of high-purity single crystals in fiber form) of silicon carbide are known to have great strengths. It would be desirable to use such whiskers to reinforce ceramic composites. However, incorporating such whiskers into ceramic composites has been difficult because blending the whiskers with the ceramic powder followed by cold pressing, hot pressing, or extruding will severely damage the whiskers and reduce their reinforcement ability.
On the other hand, graphite and carbon can be easily formed into a wide variety of shapes and sizes (e.g., by machining, by extruding precursors of graphite, and by other means). Therefore, it would be highly desirable to be able to use the graphite and carbon shapes as forms and to convert those forms to silicon carbide, with the silicon carbide conversion extending to any chosen depth within the original carbon or graphite structure. It would also be very desirable to obtain whisker-reinforced ceramic matrix composites in which the integrity of the whiskers has been preserved.
It is known that silicon monoxide (i.e., SiO) and carbon (C) react to form silicon carbide (SiC). It has been generally known that a thin surface layer of SiC has formed as a by-product in various reactions. During the growth of SiC whiskers on a carbon substrate in which SiO and CH.sub.4 gases are present, not only are SiC whiskers grown at each catalyst site but most of the surrounding areas of carbon are surface converted to SiC by the presence of low concentrations of SiO gas in the atmosphere about the carbon substrate.
To date, attempts to obtain economically good ceramic structures made of SiC have included forming a mixture of powder of carbon or graphite with powdered silicon and then heating that mixture, so as to melt the silicon in the presence of the carbon so they will react to form silicon carbide. This procedure, however, has the drawback of forming a silicon carbide material with a high degree of porosity since the mixture of pressed powders has pores; and the conversion process is not a densification process by the addition of new matter (i.e., only old material is reacted to change form). Furthermore, the articles of manufacture produced by this process may not be formed completely of silicon carbide because of inadequate mixing and dispersion of the initial silicon and carbon powders or because they may not have been held long enough to complete the diffusion. Also, as the SiC is formed, the articles shrink because the bulk volume of SiC is less than that of carbon or silicon alone.
Another procedure is disclosed in De Bacci et al., "Preparation for Storage of Fission Products," U.S. Pat. No. 3,994,822. In this process, objects made of carbon or graphite are situated within a bath of liquid silicon and form silicon carbide articles of manufacture. This process would involve difficulties in working with a large volume of hot liquid metal in an inert atmosphere, and it is believed that the liquid would not have great penetration ability into fine pores due to its liquid viscosity and rate of reaction. Furthermore, the liquid would react mostly with the surface.
Other attempts to obtain good ceramic structures economically have included coating materials with silicon carbide by decomposing a silane, CH.sub.3 SiCl.sub.3, (as disclosed, for example, in L. Aggour et al. "CVD of Pyro-Carbon SiC, TiC, TiN, Si, and Ta on Different Types of Carbon Fibers," Carbon, 1974, vol. 12, pp. 358-362) and by chemical vapor deposition reactions (such as are disclosed in Bauer, U.S. Pat. No. 3,991,248, in Bourdeau; U.S. Pat. No. 3,369,920; and in Wainer, U.S. Pat. No. 3,269,802). In such reactions, however, the external volume of the object being coated increases as the coating reaction proceeds. Furthermore, as the thickness of the coating increases, problems of bonding the coating to the substrate increase. This is in contradistinction to a conversion process wherein the substrate itself is converted, rather than merely coated.
Therefore, despite what has been known in the prior art, a need has existed until now for a method of easily and economically making silicon carbide structures of the same sizes and shapes as the sizes and shapes in which graphite and carbon can be obtained.